1. Field of the Invention
The present invention relates to a process for preparing Nb superconducting material, and more specifically to a process for preparing bulk NbN superconducting material directly.
2. Description of Related Art
The critical current density J.sub.c of alloy superconductors such NbTi and of A15 type composite superconductors such as Nb.sub.3 Sn and V.sub.3 Ga is only about 10.sup.5 A/cm.sup.2 (at 8 T, 4.2 K). On the contrary, Nb which is a kind of B1 type composite superconductors has such relative high critical temperature T.sub.c as 17 K among known superconductors except high-T.sub.c oxide superconductors and has the highest critical current density J.sub.c of 1.times.10.sup.7 A/cm.sup.2 (0 T) and 2.times.10.sup.6 A/cm.sup.2 (20 T) of all known superconductors. Furthermore, NbN has a good resistance to radiation or neutron flux, which has hundredfold improvement as compared to A15 type superconductors. NbN has also such high superconducting magnet properties that the upper critical magnetic field H.sub.c 2 measured with a anisotropic thin film of NbN is 43 T. On the other hand, NbN has not only excellent superconducting properties but also shows high heat resistance, hardness and mechanical strength.
Because NbN is so hard that it is hardly processable, NbN superconductor has been used in a form of thin films prepared by sputtering or other methods. However, there is such tendency that the critical current density J.sub.c decrease with increase of film thickness.
NbN superconductor is expected to be used in wires of solenoid of a nuclear fusion reactor owing to its good superconducting properties and high mechanical strength.
However, it was impossible to produce a bulk NbN superconductor. Here to fore, a NbN superconductor wire was produced by depositing a thin film of NbN on a core metal.